1. Field of the Invention
The present invention relates to heat retentive food servingware. The preferred servingware of the present invention comprises a heat retentive core including a solid-to-solid phase change material, a resilient material permitting expansion of the phase change material, and an inductive heating element for temperature regulation of the phase change material. The servingware of the invention is capable of temperature self-regulation when heated by a magnetic induction cooking device.
2. Description of the Prior Art
Many food preparers require use of devices for keeping food warm prior to serving and during a meal. Such preparers include institutional food preparers and servers, restaurants, caterers, individual consumers, etc. Institutional food servers such as hospitals, nursing homes, and other similar operations, commonly require a time period between food preparation and serving that can exceed thirty minutes.
Various heat retentive serving devices for keeping food warm until the food can be served are known in the prior art. Heat retentive serving devices generally include a server base and an insulated dome for the base. The most common commercially used server base is designed to support standard dishware for holding food. Prior art examples of such a server base are shown in U.S. Pat. No. 4,246,884 to Vandas, and are available from companies such as the Seco Products Corporation, and the Carter-Hoffman Corporation. Seco Products, for example, manufactures such products under the names "System 7" and "System 9".
The server base is typically comprised of a stainless steel "pellet" or base with some type of heat storage material sealed therein, a synthetic resin underliner for insulation, and a standard ceramic dinner plate resting on the pellet. Common heat storage materials in the pellet include metals and wax.
Prior art heat retentive servers are typically used in the following manner. First, stacks of stainless steel pellets are pre-heated in an oven-type heated pellet dispenser. Simultaneously, stacks of separate dinner plates are heated in the same or a similar heated dispenser. After sufficient heat has been stored in the stainless steel pellets and dishes, the heat retentive servers are assembled during meal make-up.
During such assembly, a worker carefully removes a hot stainless steel pellet using a large suction cup. The worker wears gloves to prevent bums from the hot and highly thermally conductive metal surface. The stainless steel pellet is placed atop a plastic underliner. Next, a heated, dinner plate is placed atop the pellet. This assembly is then sent down a conveyer line where food is placed on the plate. Finally, an insulated dome is coupled with the complete base to cover the food and finalize the server assembly. The food enclosed within the server is kept warm by heat passively released from the heat storage material and by the insulative effect of the dome and underliner.
U.S. Pat. No. 3,557,774 to Kreis, U.S. Pat. No. 3,837,330 to Lanigan et al., and U.S. Pat. No. 4,086,907 to Rothschild disclose examples of server bases having some type of metal or metal alloy as a heat retentive material. Each of the devices disclosed in these references includes variations in the structure of the server base for controlling metal expansion and trapped air expansion within the server base. Although many commercial server bases with metal heat storage material are in use today, they do not keep food hot long enough for many institutional food service operations. For example, due to the storage of only sensible heat, and the low specific heat, high thermal conductivity and high density of metals, these server bases either have to be extremely massive or be pre-heated to severe temperatures to match the performance of server bases using phase change materials.
U.S. Pat. No.3,148,676 to Truog et al., U.S. Pat. No. 3,734,077 to Murdough et al., and the Vandas reference disclose examples of wax-core server bases using solid-to-liquid phase change materials as a heat storage material. These references disclose a petroleum based, carnauba, or synthetic wax having a relatively high specific heat and a relatively low melting point, such as between about 170-270.degree. F. Structural differences of the devices disclosed in these references include variations of expandable wall designs to prevent rupture of the base upon fusion/expansion of the wax and various means for improving the heat transfer from the wax to the top surface of the server base. Many wax-core server bases are used by institutional food servers today, including the above noted System 7 and System 9 devices manufactured by Seco Products Corporation. Most commercially available wax core heat retentive servers claim to keep food above 140.degree. F. for more than 30 minutes, some for longer than one hour.
Despite the widespread current use of server bases including solid-to-liquid heat retentive cores among institutional food servers, several problems exist. For example, pre-heating of the stainless steel bases takes between one and two hours in commercially available oven-type heated base dispensers, limiting the flexibility of the food service operation. Upon completion of this time and energy consuming process, workers must take the extreme caution in assembling the servers to prevent burns, as noted above.
Several alternative server designs in the prior art have addressed these problems. U.S. Pat. No. 4,982,722 to Wyatt discloses a server base with upper and lower shell walls made from a low thermal conductivity, non-metallic material. An encapsulated heat core of solid-to-liquid phase change material is disposed in the cavity. This design purports to solve the problem of potential burns when removing the server base from an oven-type heater. The required pre-heating time, however, is relatively lengthy. U.S. Pat. No. 4,567,877 to Sepahpur does address the pre-heat time problem. The Sepahpur reference discloses a heat retaining server constructed with all non-metallic materials that is designed to store heat by exposing wet sand encapsulated in its base to microwaves. However, the Sepahpur device does not address the vapor pressure problem encountered when the water therein turns to steam.
Despite prior art attempts to solve the aforementioned pre-heating and safety problems with server bases, these and many other problems with prior art heat retentive servers remain unresolved. For example, prior art heat retentive servers are bulky. In institutional serving application, the bulkiness demands large transport carts for delivery of multiple meals to patients, increasing the costs of equipment, and potentially causing undue strain on workers who deliver them. Prior art heat retentive servers require special washing treatment and special racks for proper drying. Prior art heat retentive server bases also typically comprise multiple pieces that demand extra manpower and time to assemble during meal make-up and demand excessive space to store when not in use. In addition, prior art server bases with long temperature holding times, i.e. with wax core bases, may leak molten wax from their seams during normal use. This problem presents safety hazards to institutional workers and diners.
As a result of these disadvantages, restaurants generally resort to pre-heating standard ceramic dinner plates and/or special metal dishware in cooking ovens. Restaurants also use infrared heaters to keep food warm prior to serving. These methods are relatively inefficient and time consuming. In addition, such methods result in only the outer layer of food being heated, allowing the food to cool and dehydrate significantly prior to being consumed by a patron. Other known servingware heating devices include electrically powered buffets, warming trays, and aluminum heat conductive trays heated by candles, sterno or burners.
It is desirable to have a heat retentive server that to address the problems posed for institutional food servers by prior art servers. It is desirable that a novel server not only be compatible with present commercially available pre-heating equipment, but be capable of being preheated by convenient new methods to significantly decrease preparation time, reduce manpower required, and lessen safety concerns. It is also desirable that a novel heat retentive server and novel pre-heating methods be convenient, efficient, and effective enough to open new markets for their use, i.e. restaurants, caterers and individual consumers. Finally, it is desirable to provide a novel heat retentive server having structural features, especially the heat storage material therein, that is directly transferable to all manner of other servingware for use in all market segments.
To satisfy the above desires, a solid-to-solid phase change material should preferably be used. Many such materials are known. For example, a large number of solid-to-solid phase change materials were evaluated by the National Aeronautics and Space Administration (NASA) during the 1960s as thermal capacitors to passively buffer the temperature swings experienced by earth orbiting satellites. See Hale et al., Phase Change Materials Handbook, NASA Report B72-10464 (August 1972).
Among the hundreds of phase change materials evaluated by NASA were a few materials which exhibited solid-to-solid transformations with large enthalpies. Though these solid state phase change materials were not used in space applications, extensive prior art research data quantify the thermal energy storage properties of a series of solid state phase change materials. Such solid state phase change materials have several potential advantages over the solid-to-liquid phase change materials currently used in prior art heat retentive servers. These possible advantages include less stringent containment requirements, greater design flexibility, and greater potential for efficient heat transfer to and from the phase change material.
U.S. Pat. No. 4,983,798 to Eckler et al., shows a warming device and food storage container using one type of solid-to-solid phase change material, discrete solid particles of pure polyols and polyol mixtures, as the heat storage medium. The Eckler reference discloses that these polyols are lossy at microwave frequencies, particularly at the 2450 MHz frequency of commercial microwave ovens. However, due to the low thermal conductivity of polyols, a modest amount (220 g) of pure polyol, or mixture of pure polyols, requires many hours in a conventional oven to store sufficient heat so as to trigger the solid-to-solid phase transformation throughout the material. Another disadvantage is that discrete particles hinder the ability to ensure good thermal contact with the enclosure and make it difficult to eliminate air pockets that could cause expansion problems upon heating. Furthermore, without compression, discrete particles of polyol require a large volume to store sufficient amounts of energy. Finally, discrete polyol particles will not adhere to other objects. Together, these problems prohibit discrete particles, as described by the Eckler reference, from working as an effective heat retentive core of food servingware.
A solid-to-solid phase change material alone is not enough to satisfy the desires listed above. An alternative method to pre-heat an improved heat retentive server employing a solid-to-solid phase change material is necessary. The preferred alternate heating method is magnetic induction heating. Magnetic induction heating employs alternating magnetic fields such as those produced in an induction coil to induce an electric current in a body including ferromagnetic material placed in the magnetic field. The induced current in the body creates "eddy currents" which then cause the body to undergo joule heating in direct relation to the power, I.sup.2 R, of the current through the body. The joule heating effect heats the body so that the body may be used to raise the temperature of objects in contact with the body.
The use of magnetic induction as a means of pre-heating an improved heat retentive server allows an important feature not exploited in prior art heat retentive servers. That feature is temperature self-regulation without the need for thermal contact between the server and the magnetic induction heating device. Many commercially available magnetic induction cooking ranges have temperature controls that allow regulation of the temperature of a cooking utensil's bottom surface when the surface is in direct contact with the support surface of the cooking range. Typically, this is done via a feedback circuit using a transducer attached to the underside of the magnetic induction cooktop. By employing a magnetic induction heating element within the server itself that acts as an impedance switch at a designated temperature, in conjunction with the employment of a current limiting switch inherent in today's magnetic induction cooking devices, a novel heat retentive server may be constructed that is temperature self-regulating without direct heating of its bottom surface.
Temperature self-regulating magnetic induction heating elements are known and have been used in furnaces and electric soldering equipment. The following discussion highlights the theory behind these prior art elements. When a ferromagnetic metal reaches or exceeds a critical temperature, referred to as the Curie temperature, T.sub.c, the relative magnetic permeability, .mu..sub.r, of the material drops rapidly from a value of between about 100 and 1000, depending upon the metal or alloy, to a value of about 1. This automatic, reversible, switch-like change in relative magnetic permeability directly affects the concentration of induced eddy current flow in a ferromagnetic heating element. Induced eddy currents flow primarily along the surface of the element with the induced current density, j(x), decreasing exponentially as a function of the distance from the surface of the element, x. This exponential relationship between current density, j(x), and the distance from the surface of the heating element, x, is given by Equation 1: EQU j(x)=j.sub.0 e.sup.-x/.delta. ( 1)
where j.sub.0 is the current density at the surface of the element, and .delta. is a property dependent upon the material composition of the element known as the skin depth. The larger the skin depth of a particular heating element, the less concentrated the induced current is at the surface of the element. The skin depth .delta., in mks units, is given by Equation 2: EQU .delta.=(2.rho./.omega..mu.).sup.1/2 ( 2)
where .omega. is the angular frequency of the applied field in seconds.sup.-1, .rho. is the electrical resistivity of the element in ohm-m, and .mu. is the magnetic permeability of the element. It is convenient to talk in terms of the relative permeability, .mu..sub.r, where .mu..sub.r is the permeability normalized to the magnetic permeability of vacuum, .mu..sub.v, where .mu..sub.v equals 4.pi..times.10.sup.-7 Wb/A-m. Thus, .mu..sub.r =.mu./.mu..sub.v =.mu./4.pi..times.10.sup.-7 Wb/A-m. For non-magnetic materials, .mu..sub.r =1.
Now assume that the frequency and the magnitude of the induced current in the induction heating element are kept constant (by regulating the frequency and current in the primary winding of the magnetic induction heating device). Below the Curie temperature, the relative magnetic permeability, .mu..sub.r, of the heating element is relatively high. Therefore, the skin depth of the element is small. Prior to the temperature of the heating element reaching the Curie temperature, the induced current flowing through the element is highly concentrated in the surface region of the element. This high concentration provides a relatively small path for the current flow through, increasing the resistance of the element. As a result, the joule heating rate of the heating element is high and the heating element heats rapidly below the Curie temperature.
Once the element is heated above the Curie temperature, where the relative magnetic permeability of the element has dropped to 1, the induced current flowing through the heating element is permitted to spread further into the interior of the element. The resultant lower concentration of current reduces the resistance. As a result, the joule heating rate of the heating element drops significantly, enough so that the heating of the element slows. Since the ratio of maximum heating rate to minimum heating rate determines the range over which the heating element can adequately maintain constant temperature, this ratio and the corresponding ratio, R.sub.max /R.sub.min, are significant indications of the temperature self-regulation performance of the heating element.
The resistance of a heating element strip one unit wide, one unit long, and one skin depth thick is: EQU R.sub.surface =.rho./.delta. (3)
Substituting for .delta. from Equation 2: EQU R.sub.surface =(.omega..mu..rho./2).sup.1/2 ( 4)
R.sub.surface is called the surface resistivity and may be considered as the effective AC resistivity of a material. Since achieving the most rigid temperature self-regulation requires achieving the highest ratio of R.sub.max /R.sub.min, we find from using Equation 4 that this means achieving the highest ratio of: ##EQU1##
Unfortunately, commercially available magnetic induction cooking devices do not employ circuitry to maintain induced current within a load at nearly constant levels as the load's magnetic permeability drops precipitously, a premise upon which the prior art heating elements described above depend. The term constant current refers to the following relationship: ##EQU2##
Fortunately, commercially available magnetic induction cooking devices do employ circuitry designed to prohibit excessively high currents from flowing through the inverter circuit, and hence through the load. This type of circuitry, typically called a "no load" or "abnormal load" condition detector, is designed to employ a feedback parameter that depends directly upon the impedance of the load. This feedback parameter, whose detection and use do not require thermal contact with the load, and the no load detection circuitry are used to interrupt a sustained current through the induction heating coil, thus interrupting the magnetic field and protecting the inverter from the abnormal load condition, when no load or a relatively low load situation is encountered. U.S. Pat. Nos. 3,978,307 to Amagami et al., and 4,115,676 to Higuchi et al., incorporated by reference, disclose no load circuitry. Prior art servingware, however, are not provided with heating elements configured for using the no load detection circuitry to achieve temperature self-regulation.